This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science. The blog is on holiday until the start of October, when I’ll return with fresh material.

As a species, we value fair play. We’re like it so much that we’re willing to eschew material gains in order to punish cheaters who behave unjustly. Psychological games have set these maxims in stone, but new research shows us that this sense of justice is, to a large extent, influenced by our genes.

When it comes to demonstating our innate preference for fair play, psychologists turn to the ‘Ultimatum Game‘, where two players bargain over a pot of money. The ‘proposer’ suggests how the money should be divided and the ‘receiver’ can accept of refuse the deal. If they refuse, neither player gets anything and there is no room for negotiation. In a completely rational setting, the proposer should offer the receiver as little as possible, and the receiver should take it – after all, a very little money is better than none at all.

Of course, that’s not what happens. Receivers typically abhor unfair offers and would rather that both parties receive no money than accept a patronisingly tiny amount. Across most Western countries, proposers usually offer the receivers something between 40% and 50% of the takings. Any offers under 10% are almost always rejected.

The uniformity of responses across Western countries suggests that culture has a strong effect on how people play the game, but until now, no one had looked to see how strongly genes asserted their influence. Bjorn Wallace and colleagues from the Stockholm School of Economics decided to do just that, and they used the classic experiment for working out heritability – the twin study.

The twin study relies on the fact that identical twins share all of their genes, while non-identical twins share only 50%. If a personality trait or physical characteristic has a strong genetic component, it should be more similar in identical twins than in non-identical ones.

As it happens, Sweden has the largest registry of twins in the world, providing Wallace with perfect fodder for his experiments. Using the registry, he recruited 329 pairs of twins. Each person played the Ultimatum game twice, once as proposer and once as receiver, and each time with a random partner.

Wallace found that identical twins were indeed more likely to play the same strategy as non-identical twins. By analysing the results of the games, Wallace worked out that genetic effects account for 42% of the variation in how people respond to unfair offers and if anything, he feels that this is an under-estimate.

That’s not an unexpected result. According to recent studies, the responder’s behaviour is determined by brain activity in their dorsolateral prefrontal cortex, an area involved in thought and emotion. It’s also affected by levels of testosterone in their blood (and that applies to both men and women). Both of these factors have strong genetic influences, so it comes as no surprise that the resulting behaviour does too.

The way we deal with injustice is now one of a number of personality traits that are influenced by our genes. Obviously, the usual caveats apply – this doesn’t mean that our feelings about fairness are pre-determined or always the same.

When I first read about games of this kind, I was puzzled at the range of responses. I now realize that my impulse to punish cheats overwhelms my self-interest. I would accept half or nothing. I actually think I see it as a duty to punish, a civic duty, like calling attention to cruelty or rudeness.

As someone carrying a diagnosis of Severe Inattentive ADD and virtually no urge to punish perceived injustice I’m intrigued by the notion that both involve the dorsolateral prefrontal cortex.
I’d love to investigate the possible correlation between lower than average sense of “fair play” and poor executive function.

Nathan – the study found that pairs of identical twins are more likely to use the same strategy than pairs of non-identical twins. The study probably assumes that cultural differences affect both types of twins equally.

I’m not sure the assumption that cultural differences affect both types of twins equally is valid. I find it plausible that growing up with an identical twin, someone who is essentially identical to you but not you, could have a unique effect on one’s social psychology.
Ed’s article doesn’t mention whether any twins who had been separated at birth were included in the study. That would be the only way to control for such a phenomenon.

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Phenomena is a gathering of spirited science writers who take delight in the new, the strange, the beautiful and awe-inspiring details of our world. Phenomena is hosted by National Geographic magazine, which invites you to join the conversation. Follow on Twitter at @natgeoscience.

Ed Yong is an award-winning British science writer. Not Exactly Rocket Science is his hub for talking about the awe-inspiring, beautiful and quirky world of science to as many people as possible.
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